Conventionally, in a chemical plant or petrochemical plant comprising multiple equipments and apparatuses disposed on a vast site, industrial measuring instrumentation is applied for measuring physical quantities such as the pressures and temperatures in the equipments and apparatuses and the flow rates of fluids flowing through pipes, in order to control the plant. An industrial measuring instrument used in such applications is shown in FIG. 12, by way of example. The industrial measuring instrument 1 is connected to a two-wire transmission line 2 in series with a direct-current power supply 3 (e.g., 24 V) and a load resistor 4 (e.g., 250Ω), and sends the physical quantity measured thereby to the transmission line 2 after converting the measured quantity to an electric current value falling within a range of 4 mA to 20 mA.
In plants, an electric current loop circuit as shown in the figure is generally employed in which the physical quantity measured by the industrial measuring instrument 1 is converted to an electric current value within a predetermined range and then transmitted. This is because, in the case of plants, a transmission cable with a large length is used and thus, if the measured quantity is transmitted by means of voltage amplitude, the transmission itself is likely to fail due to signal degradation attributable to the electrical resistance of the cable.
In the electric current loop circuit, the industrial measuring instrument generally outputs an electric current proportional to the measured physical quantity within the range of 4 mA to 20 mA, wherein 20 mA represents a 100% physical quantity that the measuring instrument can measure and 4 mA represents a 0% physical quantity. The load resistor has a resistance of 250Ω because voltage drops of 1 V and 5 V occur when 4 mA and 20 mA currents flow through the resistor, respectively, and the voltage range of 1 V to 5 V is convenient for various control actions and computations.
As industrial measuring instruments of this type, a two-wire transmitter disclosed in Unexamined Japanese Patent Publication No. H09-81883, for example, is known in which an industrial measuring instrument (hereinafter merely referred to as measuring instrument where appropriate) itself generates operating power by using the electric current flowing through a two-wire transmission line, thus making it unnecessary to use a power supply unit and a power supply line for operating the measuring instrument, or to use a battery. This two-wire transmitter generates operating power therein by using the electric current flowing through the two-wire transmission line and outputs, to the two-wire transmission line, an electric current corresponding to the physical quantity measured by the measuring instrument.
The physical quantities measured by individual measuring instruments are supplied through the respective two-wire transmission lines to a centralized control device for controlling the plant, for example. The centralized control device performs various processes, such as corrections and calculations, on the data obtained through the two-wire transmission lines and controls the entire plant on the basis of the processed data. In plants with complex control systems, diverse outputs from different measuring instruments need to be handled. Accordingly, users who operate or monitor plants with such complex control systems are demanding to monitor each of numerous measuring instruments in the plant in order to determine whether the output data of the individual measuring instruments is accurate or not and what trend the output data shows, for purposes of precautions and maintenance.
Also, in cases where the measurement data outputted from the two-wire transmitter is used for purposes other than the control by the centralized control device for controlling the entire plant (e.g., in the case of monitoring the electric currents flowing through the transmission lines), it is sometimes necessary that programs in the control loop or the system be modified. It is, however, difficult to extract data of the individual measuring instruments, out from the programs used in the existing control system, to be reused for monitoring, especially in cases where the measurement system is large in scale and complicated. In some plants, therefore, measures are taken as an effective means such that data outputted from the individual measuring instruments is collected through separate lines for monitoring, independent of the transmission lines (two-wire transmission lines) used for control purposes. Generally, the value measured by an electric current monitoring device is sent through a transmission line (wired transmission path, e.g., GPIB) laid separately from the two-wire transmission line.
The electric current monitoring device may alternatively be configured to transmit the measured value over a radio transmission path, as shown in FIG. 13, for example, instead of the wired transmission path. With this method, the electric current monitoring device measures the electric current flowing through the two-wire transmission line, and the measurement data is radio-transmitted by a wireless monitoring device 5. The wireless monitoring device 5 is provided with a battery 6, for example, as a power supply for operating the device 5. This type of industrial measuring instrument with a battery-driven electric current monitoring device using a radio transmission scheme is also commonly known in the art.
In order to transmit the electric current value measured by the electric current monitoring device, however, it is necessary that a transmission line be laid separately from the line used for control purposes, requiring a transmission line with a substantially large length. Further, where the electric current monitoring device requires a power supply, a power supply line needs to be extended to the electric current monitoring device over a distance almost equal to the length of the transmission line. Thus, if the electric current monitoring device is distant from the centralized control device, that is, if the power supply line needs to be extended over a long distance, costs increase considerably.
The power supply line can be omitted if the electric current monitoring device is driven by a battery. Also, if the electric current monitoring device is configured to transmit the measured electric current data by wireless and to operate on a battery, it is unnecessary to use a power supply line or a transmission line. However, battery has a limited lifetime and must be replaced. Thus, the battery-driven electric current monitoring device requires the labor of replacing the battery. Further, since a large-scale plant usually includes a large number of electric current monitoring devices, much labor is required to locate the battery-driven electric current monitoring devices within the site of the plant. Also, if the batteries of the electric current monitoring devices are replaced with new ones whenever the batteries go dead, the replacement work entails substantial costs. It is therefore preferable that the batteries of the electric current monitoring devices be collectively replaced at regular intervals before the batteries go flat. With this method, however, there is a possibility that the batteries will be replaced at unduly short intervals, and a problem arises in that usable batteries also are discarded, which leads to waste of resources. In the case of the battery-driven electric current monitoring devices, moreover, there are restrictions on locations where the electric monitoring devices are to be arranged, because of their need for battery replacement.
As an alternative, the electric current flowing through each two-wire transmission line may actually be measured by a maintenance person with the use of a digital multimeter or tester. It is necessary, however, to provide a power supply (commercial power supply or battery) for operating the digital multimeter or the like, and a problem also arises in that real-time measurement is not available.
In the two-wire transmitter (hereinafter merely referred to as transmitter) disclosed in the aforementioned patent document, a supply voltage required to drive the transmitter is generated by a shunt regulator using the electric current flowing through the two-wire transmission line, and then the electric current flowing through the transmission line is adjusted to an electric current value corresponding to the value detected by a sensor provided in the transmitter. Thus, although the electric current flowing through the two-wire transmission line is controlled by the transmitter, the transmitter does not detect an actual electric current value of the transmission line. This will be explained in more detail with reference to the circuit diagram of FIG. 1 of the patent document. The electric current flowing through the transmission line is controlled by using the terminal voltage of a feedback resistor R2. Accordingly, even if the electric current flowing through the two-wire transmission line deviates from a desired value due to degradation of resistors (R5, R6, R7, R8, R2) or transistors (Q4, Q2), short circuit, etc., fault of these elements cannot be detected since the electric current flowing through the transmission line is not detected. Namely, when the measuring instrument itself is faulty, the transmitter of the patent document is controlled using the output electric current of the faulty measuring instrument. It is therefore difficult to detect fault of the transmitter itself.